Volume control for variable volume fluid pressure translating device



. NEFF ETAL 3,186,310 FOR VARIABLE VOLUME FLUID D.B VOLUME CONTROL PRESSURE TRANSLATING DEVICE June 1, 1965 Filed Feb. 18, 1963 IN VEN TORS- United States Patent 3 186,310 VGLUME CONTROL FGR VARIABLE VOLUME. FLUID PRESSURE TRANSLATING DEVICE Darby B. Neil, Worthington, and Donald F. Raider, Coiumbus, Ohio, assignors to American Brake Shoe Company, New York, N.Y., a corporation of Delaware Filed Feb. 18, 1963, Ser. No. 258,991 2 Claims. (Ci. 91-414) This invention relates to means for controlling the volumetric capacity of variable volume fluid pressure trans lating devices.

It has been a primary objective of this invention to pr vide remotely operable means for controlling the volume of fluid under pressure passing through a variable volume fluid pump or motor.

Pumps and motors having means for adjusting or changing their volumetric capacity, that is, the volume of fluid which they deliver or pass per unit time at a given operating speed, are known as variable volume pumps or motors. Such fluid pressure translating devices may be classed as either single side type or cross center type. Single side variable volume devices are adjustable to establish a volume of fluid which is variable between Zero and a maximum of flow in one direction, and are so named because their volume changing mechanism is movable from a center or zero flow position to one side of center only. Cross center (or over center) type devices are capable of adjustment to establish a maximum of flow in one direction, to zero flow, to a maximum of flow in the opposite direction. In cross center type devices the volume changing means is movable from one side of center or Zero flow position, across center, to the opposite side of center. The control means we have invented is of utility with both cross center and single side pumps and motors. For purposes of description, we have described our invention herein primarily in relation to pumps, but it is equally applicable for use with variable volume fluid motors.

In accordance with a preferred embodiment of this invention, the volume changing means of the translating device is positioned by fluid motor means which preferably comprise opposed fluid operated pistons. A control valve including a servo-spool controls the application of fluid under pressure from the source to operate the motors to change the position of the volume changing means. The spool of the control valve is mechanically linked to the volume changing means by an interconnecting mechanism whereby when the valve is actuated to change the position of the volume changing means, the movement of the volume changing means in response thereto is fed back to the spool and restores the spool to its neutral position as the volume changing means assumes the new position. The spool oi the control valve is actuated by a fluid operated rotary mechanism which is connected to it by a linkage transforming rotary movement into lineal motion of the spool of the control valve. The rotary member preferably comprises a pivotally mounted cros arm, and a plurality of fluid operated pistons are engageable with this cross arm to rotate the cross arm when any one of the pistons is operated. The stroke of each piston is limited by an adjustable cooperating stop, which thereby also limits both the rotation of the cross arm when the piston is operated and the corresponding lineal movement imparted to the control valve. Electrically controlled valves control the application of fluid under pressure from a source to the respective pistons, so that by supplying an electrical signal to the appropriate valve fluid can be applied selectively to one of the cross arm control pistons, and that piston is actuated to rotate the cross arm by an amount determined by the setting of its associated stop. The stops are preset to permit different degrees of rotation of the cross arm, corresponding to different pump volu- Patented June 1, 1965 'ice metric outputs. Spring centering means bias the cross arm toward a neutral position, in which the volume changing means is set at center position, so that when none of the pistons are operated the pump will deliver no substantial volume of fluid.

The invention may best be described in further detail by reference to the accompanying drawing, which is a diagrammatic view of a fluid power transmission system including the principles of this invention, in which the volume of fluid under pressure supplied by an overcenter or reversible output piston type hydraulic pump is controlled by an electrohydraulic volume control system.

The system shown in the drawing includes a variable volume pump 20 the volumetric capacity or displacement of which may be adjusted between a maximum of flow in one direction, to zero, to a maximum of flow in the opposite direction. The pump 20 may be a conventional pump of this type having a shaft 21 which is driven in one direction by a prime mover such as an electric motor, not shown. The pump 29 has two ports which are connected to conduits 22 and 23. Depending upon the position or setting of the volume changing means of the pump, one of the conduits 22 or 23 will constitute the pressure or outlet conduit and the other conduit 23 or 22 will constitute the inlet or suction conduit of the pump.

Conduits 22 and 23 are connected to a fluid motor 24, which for purposes of description is shown as a reversible rotary motor. The direction of rotation of the fluid motor 24 depends upon which of the conduits 22 or 23 is the pressure line and which is the suction line. The motor 24 may be employed as a prime mover for driving a machine; however, those skilled in the art will appreciate that fluid motor 24 might alternatively be a fluid operated piston, a double ended cylinder, or a hydrostatic transmission, for example.

The above described elements are connected to form what is known in the art as a closed circuit, that is, a circuit in which fluid is supplied to the motor from the pump and fluid is returned directly to the pump from the motor. While the present system may contain either a gaseous or liquid fluid, for the purposes of this description the system and all of its fluid operated or controlled devices will be described as hydraulic devices and the system will be considered as being filled with hydraulic fluid.

The cross center variable volume pump it includes a fixed port plate 25, a cylinder barrel 26 bearing thereagainst which is driven by shaft 21 and a plurality of pistons 27 which reciprocate in cylinder bores in the cylinder barrel 26. The ends of the pistons 27 bear upon a swash plate 28 which .is pivotally mounted to swing upon a pair of trunnions one of which is shown at 29. When the swash plate 2% is rotated from the position in which it is shown in the drawing, it causes the pistons 27 to reciprocate in their cylinder bores as the barrel 26 is rotated and to pump hydraulic fluid to either conduit 22 or 23 in a direction depending upon the direction of rotation of the swash plate. The position or angleof tilt of this swash plate determines the volumetric capacity or displacement of the pump, increasing angulation from the position shown effecting an increase in the volumetric capacity of the pump.

It is to be understood that while the pump 26) is referred to herein as a variable volume cross center axial piston type pump, a variable volume pump of any other type may be substituted for it within the scope of the invention. Also, as previously expressed, the invention is adapted for use with variable volume motors.

The position of the swash plate 2? of pump 29 is adjusted by fluid motor means which includes two opposed cylinder and piston type hydraulic motors -30 and 31, the respective pistons .32 and 33 of which bear against cams 34, 34 on opposite sides of an extension or arm 35 rigidly connected to the swash plate 28. Pistons 32 and 33 are urged into constant engagement with the earns 34, 34 by elastic means such as springs 36 and 37 respectively.

The area of piston 32 of fiuid motor 39 which is exposed to fluid under pressure is greater than the area of piston 33 of motor 31, and for this reason if hydraulic fluid at the same pressure is simultaneously applied to both pistons 32 and 33, the force exerted by piston 32 will be greater than the force exerted by piston 33 and Piuton 32 will move the extension 35 and swash plate 28 in a counter clockwise direction by overcoming piston 33 and spring 37.

The above described fluid motor means which position the volume changing means of swash plate 28 is under the control of a valve 47 which includes a body or housing forming a cylinder 48 in which there is a slidable core or servo spool 49. Spool 49 is grooved circumferentially at 50 to provide a pair of spaced lands 51 and 52. The cylinder 48 also includes a fluid pressure inlet port 53, which is located so as to be in constant communication with the groove 50 in spool 49, and a port 54 the opening of which is controlled by land 52. The diameter of the port 54 is preferably only slightly less than the thickness of land 52, in order that very small axial movement of the spool 49 in either direction will open the port 54 to one side or the other of land 52. The opposite ends of the spool 49 are connected to drain lines 55 and 56 which may be connected to a fluid reservoir or tank 57.

It will be seen that the spool 49 is hydraulically balanced because the areas of lands 51 and 52 at each side of the groove 50 are equal and the lands are exposed to the same pressure, and because the opposite ends of the spool are connected to the reservoir 57.

Hydraulic pressure for operating the motor means including the motors and 31 is derived from a small hydraulic pump 68 which may be driven by the motor which drives pump 29 or from a separate source of power, not shown. Pump 68 receives fluid from the tank or reservoir 57 through a suction line 69, and discharges fluid under pressure into a line 70 which is connected to a sequence valve 71, which will be more fully described hereinafter, and to lines 72 and 73. Line 72 leads to the port 53 in valve 47 and to the cylinder of motor 31. Port 54 of valve 47 is connected through a line 74 to the cylinder of motor 30.

When the system is operating, fluid pressure from the pump 68 is directed through conduit 72 to the cylinder of motor 31 and through port 53 in valve 47 to the groove 56 in spool 49. When the spool is in the position shown in the drawing, port 54 is closed or blocked by land 52, and fiuid in the cylinder of motor 30 is trapped so that the piston 32 thereof cannot be moved upwardly into its cylinder to permit the piston 33 of motor 31 to swing the swash plate 28 and change the volumetric capacity or displacement of the pump 2%.

When spool 49 of valve 47 is moved upwardly to connect port 54 with drain line 56, the cylinder of motor 30 is connected to tank 57 through lines 74 and 56, and the fluid under pressure in the cylinder of motor -31 will move piston 33 upwardly, thereby causing swash plate 28 to decrease the volumetric capacity of the pump it arm is then below center position or to increase the volumetric capacity of the pump if the arm 35 is above center position. When the spool 49 is again moved downwardly to close port 54 the swash plate 28 will be held in an adjusted position, and when the spool 49 is moved downwardly sufiiciently to cause the land 52 to connect port 54 with the groove 59, then fluid under pressure from pump 68 will be directed to both motors 30 and 31, and the arm 35 and swash plate 23 will be moved in counterclockwise direction. In the event of failure of fluid pressure from pump 63, the springs 36 and 37 hold the arm 35 in center position, and the pump 29 produces no substantial volume of fluid under pressure.

As previously mentioned fluid from the pump 68 flows through conduit 70 to the sequence valve 71. This valve 71 functions to assure that there will always be adequate pressure in the conduits 70 and 72 to operate the mot-or means 30, 31. The sequence valve 71 may be of standard construction and is shown in the drawing as comprising a body 75 and a cap 76. The body 75 includes a stepped bore 77 in which there is a stepped core 78. Core 78 includes a head portion or land 79 and a narrower piston or stem 80. The cap 76 includes a bore which is aligned axially with the stepped bore 77, and contains a spring 81 which abuts the head 79 of core 78. A movable abutment is adjusted by screw 82 to vary the compression of spring 81 and the force with which that spring urges core 79 downwardly in bore 77. The conduit 70 enters the body 75 through a port '83 which is in constant communication with a passageway 84 leading to the end of the smaller diameter portion of bore 77 and to the end of piston 80. An outlet port 85 in the body 75 is opened and closed, that is, connected and disconnected with the port 83 by the head or land 79 of core '78. Outlet port 85 is connected to a line 90, and line is connected to four lines 91, 92, 93, and 94. Line 91 includes a check valve 95 which permits flow away from but not toward port 85 and is connected to the inlet of a relief valve 96 and to conduit 22 of pump 20. Line 92 includes a check valve 97 which permits flow away from but not toward port 85 and is connected to the inlet port of a relief valve 98 and to conduit 23 of pump 20. Lines 93 and 94 are connected to the respective outlet ports of relief valves 98 and 96. Line 94 is connected through a supercharged pressure control valve 99 which, in practice, may be a spring loaded check valve permitting flow at pressures above about 50 psi. in the direction away from the drain ports of relief valves 96 and 98. The downstream side of valve 99 is connected to a drain line 100 leading to the tank or reservoir 57. A drain line 101 connects the bore in cap 76 with conduit 100.

It will be seen that the sequence valve 71 will not open its port 85 until the pressure in conduit 70 is sufficient to move the core 78 and overcome the spring 81. The fluid which then passes through the port 85 can flow through lines 91 and 92 to the conduits 22 and 23 of the previously described closed pump-motor circuit to maintain the latter filled with oil regardless of which line 22 or 23 constitutes the pressure line. The oil which cannot be accepted by the closed circuit will pass through either relief valve 96 or 98 to the tank 57.

The control valve 47 is operated through mechanism which includes a linkage system herein shown as being comprised of a lever 11!) mounted for pivotal swinging movement upon a fixed pivot point 111. One end of this lever is bifurcated to form a slot through which it is connected by a pin 112 to the end of arm 35, which is connected to swash plate 28. The other end of the lever 110 is pivotally connected to one end of a link 113 which connects it to one end of the second lever 114. The operating stem 116 of spool 49 of valve 47 is connected through a link to the lever bar 114 approximately at the center of the latter.

From the foregoing it will be seen that if the left end of lever 114 is moved downwardly with respect to its right end, this downward movement will be transmitted through link 115 and operating stem 1-16 to spool 49 of valve 47, and that land 52 will open port 54 to communicate with port 53. Fluid under pressure supplied to line 72 from pump 68 will then flow through the control valve 47 to lip e 74, to fluid motor 30 and will move the piston 32 thereof downwardly, against the lesser force of opposing piston 33, thereby swinging arm 35 downwardly from the position shown, so as to increase the volume of fluid delivered by the pump 20. As arm 35 is moved downwardly by piston 32, its motion is transmitted through pin 112 to lever 110, causing that lever to pivot clockwise about its pivot point 111. If the left end of lever 114 does not move vertically upward, movement of link 113 resulting from the clockwise movement of lever 119 will move the right end of lever 114 upwardly, thereby lifting spool 49. When arm 35 has been moved downwardly sufiiciently far to cause spool 49 to close port 54, fluid is blocked from flowing into line 74 and arm 35 is held in its then position until another movement is imparted to the left end of lever 114. a

When the left end of lever 114 's moved upwardly from the position shown in the drawing, spool 49 is raised so that port 54 communicates with drain line 56, and fluid above piston 32 in motor 36 is released to drain through line 74 while fluid under pressure acting on piston 33 of motor 31 swings arm 35 upwardly. As arm 35 is raised from the position shown in the drawing, link 113 is moved downwardly, thereby moving the spool 49 downwardly, =until port 54 is again closed. For any given steady position of arm 35, fluid is blocked from flowing into or out of the chamber of fluid motor 30, and the position of the swash plate is maintained hydraulically; Under these conditions, the entire output of pump 68 (except for leakage fluid supplied to the closed circuit) is discharged to tank 57 through relief valves 96 and/ or 98.

Thus, the fluid motors 30 and 31, control valve 47 and the linkage connecting the operating stem of that valve ,to the arm 35 will be seen to comprise a rotary servo control for the volume changing means of pump 29, whereby .rotational movement imparted to the left end of lever 114 is elfective to adjust the volumetric capacity of the pump.

Electrically controlled fluid actuated mechanism positions the left end of lever 114 to provide a plurality of preselected volumetric outputs from pump 20. This volume control mechanism includes a rotatable valve actuator 120 which turns about a fixed pivot point 121 and which is shown for purposes of description as comprising five integrally connected cross arms designated as 122, 123, 124, 125 and 126. Arm 126 of valve actuator 120 is connected to the left end of lever 114 through a link 127.

It will be seen that clockwise motion of the valve actuator 120 about point 121 moves the left end of lever 114 upwardly, thereby raising spool 49 of valve 47, and that counter clockwise rotation of valve actuator 120 moves spool 49 downwardly in valve 47. Thus,'the flow of fluid supplied by the pump 20 is controlled by the direction and extent of rotation imparted to valve actuator 120.

Adjustably limited rotation of valve actuator 120 in different amounts and directions is provided by a plurality of fluid operated valve actuator positioner motors 'or pistons, six such pistons being shown in the drawing for purposes of illustration, designated as 131, 132, 133, 134, 135 and 136. Each of these piston motors 131- 136 includes a fluid pressure operated piston 138 which is biased against the actuating fluid pressure by a spring 139. The motors 131-136 are mounted by suitable means not shown so that their respective pistons will engage the arms 122-125 when fluid under pressure is applied selectively to them. The arms 122-125of valve actuator 120 are provided with apertures or bores 140 in alignment with the pistons 138 of the respective fluid motors 131-136, and the holes 140 are of slightly greater diameter than the pistons 138 so that the pistons will pass freely through :each corresponding hole 140. A shoulder 141 of diameter greater than that of the opposite hole 140 is formed on each piston 138 so that when that piston is actuated by fluid under pressure, the face of shoulder 141 abuts the surface of the arm 122-125 thereby causing the rotatable valve actuator 12% to turn and 156 which bear against the springs.

centered three-position four-way valve.

the valve actuator upon actuation'of any one of the fluid motors 131-136, and thereby determine the corresponding flow delivered by the pump 20. The diameter of each stop 142-147 is smaller than the diameter of the corresponding hole 140, so that the stops themselves do not come into contact with the arms 122-126 as the latter rotate but rather restrict their rotation by limiting the strokes of the pistons 138 byendwise engagement with them. 7 a V A pair of opposed springs 151 and 152 bear upon opposite sides of one of the arms and bias it toward center or neutral position, in which land 52 of control valve 47 just closes port 54 and arm 35 is in neutral position. The compression of springs 151 and 152 is adjusted by a pair of threaded abutment members 153 and 154 respectively, having shoulders 155 Springs 151 and 152 provide fail-safe operation of the control mechanism, in that if hydraulic control power is lost the valve actuator 120,is automatically moved to center or neutral position.

Fluid under pressure for operating the fluid motors 131-136 is supplied from pump 68 through line 73. Fluid in line 73 is selectively applied to the motors 131- 136 through three solenoid operated four-way valves 159, and 161. Each of the valves 159-161 is a spring Each valve has a pressure port 162 which is connected to line 73, to which fluid under pressure is constantly delivered by pump 68. Each valve also has a tank port 163 which .is connected to drain line 100 leading to tank 57 as previously described. Each valve also has a pair of outlet ports 164 and 165. Port 164 of valve 159 is connected to fluid motor 131 by a line 170, and port 165 of valve 159 is connected to motor 132 by a line 171. Port 164 of valve 160 is connected to motor 133 by a line 172, and port 165 of valve 160 is connected to motor 134 by a line 173. Port 164 of valve 161 is connected to fluid motor 135 by a line 174, and port 165 of valve 161 is connected to motor 136 by a line 175. Each line 17 0-175 includes a flow restrictor or throttling orifice 176, the function of which is to prevent surges of flow and which have been found to generally smooth out the operation of the mechanism.

is shifted and port 162 is connected to line 171 so that fluid under pressure is supplied tofluid motor 132. Line is then connected to pressure port 163, and fluid motor 131 is connected to tank line 100. When sole mold 177 of valve 159 is energized, the connections are reversed, port 162 is connected to line 170, fluid under pressure is supplied to fluid motor 131, and line 171 is connected to drain line 100. ,The operation of the other valves 160 and 161 is similar.

Actuation of any one of the six solenoids of the three valves 159-161 causes the pump 20 to deliver a volume of fluid which is determined by the setting of the stop of the fluid motor 131-136 to which fluid under pressure is being delivered. For example, if solenoid 178 of valve 161 is energized, the other solenoids being deenergized, fluid under pressure from line 73 is applied downwardly, causing fluid under pressure to be supplied to fluid motor 30, thereby swinging arm 35 downwardly by an amount corresponding to the movement imparted to arm125, and causing'pump 20 to deliver a predetermined volume of fluid. During this sequence the pistons 138 of the other motors 131-135 are not operated and do not limit the movement of the valve actuator 120. It will be noted, however, that if two or more of the motors 131-136 are operated simultaneously the actuator will move insofar as permitted.

The stops 142-147 are set to provide difierent desired outputs upon operation of their cooperating pistons 131- 136 respectively. Thus, for example, if the pump is capable of delivering a maximum output of 20 gallons per minute, stops 142, 144, 145 may be set to provide flows of 5, 10 and 20 gallons respectively, in one direction, while stops 143, 146 and 147, being oppositely oriented with respect to the cross arm 120 may be set to provide flows of say 2, 8 and g.p.m. in the opposite direction. The springs 151 and 152 operate the System to deliver zero output when none of the pistons 131-136 are operated.

The rotary cross arm 120 and servo mechanism comprising valve 47, motors 30 and 31, and the associated linkages together form a rotary servo mechanism for controlling the position of the volume changing means of the pump. Step-form inputs to this system are provided by the pistons 131-136.

The mechanism we have invented is not only suitable for use with a cross-center pump of the type shown, wherein the fluid motors are arranged to provide positioning of the pump volume changing means on both sides of center, but it can also be used with a single side translating device or a device which is operated as a single side device, by arranging the motors to provide positioning of the volume changing means on one side of center only.

While it is convenient for compactnessto form the valve actuator with a plurality of arms, as shown, it is of course possible to use a rotatable member having a single surface against which all of the fluid motor pistons bear.

The solenoids of the four-way valves 159-161 may be manually operated, by manually closed switches, or may be actuated by mechanically actuated switches, for example by switches which are actuated by the ram of a press which is operated by the pump 20. Alternatively, the inputs to the operating controls of the valves 159- 161 may be programmed, for example, on a tape.

It is contemplated that the pressure translating device and its associated volume control mechanism, tank, and secondary pump 68 may be manufactured and sold as an integral unit or package, which can be connected to any system in which fluid under pressure is to flow at different flow rates.

Having described our invention, what we claim is:

1. Control means for a fluid pressure energy translating device of the type having movable means for changing its volumetric capacity, said control means comprismg:

fluid motormeans for adjusting the position of said movable means,

a lineally actuable servo control valve mechanism controlling the operation of said fluid motor means in accordance with the lineal actuation of said mechanism,

and fluid operated remotely controllable means for lineally actuating said control valve mechanism, said remotely controllable means comprising,

a rotatable cross arm having outwardly extending arms the application of force to which causes said actuator to rotate,

a plurality of fluid operated pistons mounted to rotate said cross arm when actuated,

an individually adjustable stop limiting the stroke of each said piston and thereby limiting the rotation of said cross arm in response thereto,

said arms having openings therethrough in alignment With said pistons through which the ends of said pistons pass when said pistons are actuated, each piston having an abutment thereon of diameter larger than said opening to abut said arm and rotate said arm upon actuation, each stop being mounted on the opposite side of the arm from its cooperating piston to abut the end of said piston extending into said aperture when said piston is actuated,

a solenoid controlled valve associated with each piston for controlling the application of fluid under pressure thereto,

and means translating rotation of said cross arm in response to actuation of any of said pistons into lineal actuation of said control valve mechanism.

2. A volume control for a fluid pressure energy translating device of the type having movable volume changing means the position of which determines the volume of fluid passed by said device, said control comprising,

fluid operated servo means for positioning said volume changing means to adjust the volume of fluid passed by said device,

a rotatable servo actuator,

a plurality of actuator positioner fluid motors, each said motor being mounted to impart rotation to said actuator upon operation thereof,

adjustable stop means associated with each said motor for limiting the movement of said motor upon operation thereof,

each said motor having an actuator abutting portion which engages said actuator upon operation of said motor to impart rotation to said actuator, each said motor also having a stop means engaging portion, said actuator having openings therethrough corresponding to the positions of the respective stop means, each corresponding stop means, opening in said actuator, and stop means engaging portion being aligned to permit engagement of said stop means and stop means engaging portion through said opening to limit the movement of said actuator upon operation of the corresponding motor,

a valve associated with each said particular fluid motor for controlling the operation thereof,

and link means translating rotary movement of said actuator imparted thereto by said fluid motors into actuation movement of said servo means.

References Cited by the Examiner UNITED STATES PATENTS 3,065,740 11/62 Wiedmann et al 103-38 LAURENCE V. EFNER, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Darby B1 Neff et alt It is hereby certified that error a ent reqliring carrectio'n and that the correctedbelow.

ppears in the above numbered patsaid Letters Patent should read as Column 8, line 53, for "particular" read positioner Signed and sealed this 12th day of October 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Altosting Officer Commissioner of Patents 

1. CONTROL MEANS FOR A FLUID PRESSURE ENERGY TRANSLATING DEVICE OF THE TYPE HAVING MOVABLE MEANS FOR CHANGING ITS VOLUMETRIC CAPACITY, SAID CONTROL MEANS COMPRISING: FLUID MOTOR MEANS FOR ADJUSTING THE POSITION OF SAID MOVABLE MEANS, A LINEALLY ACTUATABLE SERVO CONTROL VALVE MECHANISM CONTROLLING THE OPERATION OF SAID FLUID MOTOR MEANS IN ACCORDANCE WITH THE LINEAL ACTUATION OF SAID MECHANISM, AND FLUID OPERATED REMOTELY CONTROLLABLE MEANS FOR LINEALLY ACTUATING SAID CONTROL VALVE MECHANISM, SAID REMOTELY CONTROLLABLE MEANS COMPRISING, A ROTATABLY CROSS ARM HAVING OUTWARDLY EXTENDING ARMS THE APPLICATION OF FORCE TO WHICH CAUSES SAID ACTUATOR TO ROTATE, A PLURALITY OF FLUID OPERATED PISTONS MOUNTED TO ROTATE SAID CROSS ARM WHEN ACTUATED, AN INDIVIDUALLY ADJUSTABLE STOP LIMITING THE STROKE OF EACH SAID PISTON AND THEREBY LIMITING THE ROTATION OF SAID CROSS ARM IN RESPONSE THERETO, SAID ARMS HAVING OPENINGS THERETHROUGH IN ALIGNMENT WITH SAID PISTONS THROUGH WHICH THE ENDS OF SAID PISTONS PASS WHEN SAID PISTONS ARE ACTUATED, EACH PISTON HAVING AN ABUTMENT THEREON OF DIAMETER LARGER THAN SAID OPENING TO ABUT SAID ARM AND ROTATE SAID 